build: build and publish docker image on release

[mueckinger]

Hi! Finally I figured out how to automatically build docker images on every new rustic release.
Closes #1083
Completes tasks discussed in #969, #1008 and #1010.

Notes to the maintainers:

• This requires setting Workflow permissions to write access for the GITHUB_TOKEN as described here: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/enabling-features-for-your-repository/managing-github-actions-settings-for-a-repository#configuring-the-default-github_token-permissions
• It makes sense to update the README (added docker install info) in the next rustic release (when the first image is built)

[simonsan]

Thank you! 🚀